The present invention relates to a hydrostatic steering system for motor vehicles.
Vehicle steering apparatus of the type to which the present invention relates, is generally known as disclosed for example in U.S. Pat. No. 4,553,389 to Tischer et al. In such steering apparatus, a controlling pump and/or a flow dividing valve supplies pressure medium to a steering servo motor for meeting instantaneous steering requirements. The demand for pressure medium is signaled by a differential pressure across a metering orifice that regulates the supply of the pressure medium. The pressure supply in such prior art systems is such that the differential pressure at the metering orifice not only breaks down when the main pump goes out of action, but also when the main pump is intact and the steering speed is so high as to require the entire output of the main pump for steering purposes, under engine idling conditions for example. Under such an operating situation, involving rapid steering movements under engine idling conditions, a reserve pump is utilized to augment the main pump output. However, under slow vehicle travel conditions, the reserve pump customarily driven by the vehicle wheels cannot satisfy the demand for flow of pressure medium. Excessive wear of the reserve pump therefore results from the pump acceleration effort encountered.
It is therefore a basic object of the present invention to eliminate the aforementioned disadvantages of known hydrostatic auxiliary steering apparatus and to improve the same in such a manner that the reserve pump is only enabled and loaded when a specific minimal steering speed cannot otherwise be achieved with the flow from the main pump.